This invention relates to a functional sound insulation resin composition useful against airborne-sound and structure-born sound, or more particularly to a sound insulator with absorbent and damping performance, composed of a polymeric material which comprises a polymer colloid, hardener, and an acoustic activity agent. The resin composition can be shaped into any desired form such as pellet, thread, sponge, rubbery sheet or hardened plate, and, like usual polymeric substances, it can be freely cut and bonded by an appropriate cement.
Conventional sound adsorbents for air-born sound make use of the frictional resistance of bubbles contained in porous materials. The disadvantages of those adsorbents are bulky volume, low absorbency and very poor efficiency for insulation agaist sound traveling from one room to another. While prior known sound insulators have been developed which place an emphasis on enhancing their density characteristics, such insulators are usually of heavy construction and poorly dissipate the sound energy. A compact and less heavy insulator with sufficient absorption has not yet been provided because of a tremendous difficulty to achieve a high loss material at an audio frequency especially below 1 KHz.
The dissipation process in an amorphous phase consists of two factors of shear and compressional losses in a homogenous body; the high shear absorbency is easily attainable while the large bulk absorbency is difficult to achieve.
With incident intensity I.sub.i and transmitted intensity I.sub.t of a sound wave impinging normally on a single viscoelastic plate of thickness d, mass m per unit area, and attenuation constant .alpha., the transmission loss TL is given by TL=(I.sub.i /I.sub.t)10.alpha.d/10. When .alpha..apprxeq.0 as in the conventional case, TL.sub.O =I.sub.i /I.sub.t is roughly given by the mass law. On the other hand, if .alpha. is large enough and comparable to the mass law loss, TL at the normal incidence is given by the equation EQU TL=TL.sub.O+.alpha. d,
where TL.sub.O= 20.multidot.log.sub.10 (fm)-42.5 db and f is sound frequency. Differentiating the equation for the given transmission loss, one obtains (.alpha./8.7).increment.d=-.increment.m/m which indicates that an increase of .alpha..gtoreq.d causes a decrease of the mass given by the expression-=0 .increment.ln m; the larger the absorption, the less the mass. The equation, therefore, is a guide to a quite unique approach for the insulation problem by enhancing the absorption itself, contrary to the usual techniques such as mixing heavy metallic powders to increase the loss. Assuming an upper limit d.apprxeq.3 cm, for instance, in an insulation door, a desirable attenuation would be on the order of 5 db/cm or more around 0.5 KHz.